The present invention relates to optical communication networks generally, and more particularly to optical communication networks which utilize wavelength division multiplexing.
Today, a growing demand for telecommunication services is experienced by the telecommunication industry and the demand is further expected to grow in the future. One of the ways to meet such demand is by expanding capacity of information carried over fiber optic cables in optical communication systems through use of wavelength division multiplexing (WDM).
The use of WDM enables simultaneous transmission of multiple signals at different wavelengths over the same fiber optic cable. The multiple signals at the different wavelengths are combined and transmitted in a combined form along the fiber optic cable to a receiving end. At the receiving end the multiple signals are received and separated, and the data carried over each separate channel wavelength is regenerated.
Basic elements of a WDM based system, such as wide band optical amplifiers, multiplexers/demultiplexers, optical transmitters, optical receivers and tunable optical filters, exist today and are commercially available. However, there are still several network control and management problems that have to be resolved in order to provide efficient WDM based optical communication networks.
Some aspects of technologies and art related to WDM based optical communication systems and to elements of WDM based optical communication systems are described in the following publications:
an article titled xe2x80x9cMining the Optical Bandwidth for a Terabit per Secondxe2x80x9d, by Alan Eli Willner, in IEEE Spectrum, April 1997, pp. 32-41;
an article titled xe2x80x9cRecord Data Transmission Rate Reported at ECOC 96xe2x80x9d, by Paul Mortensen, Laser Focus World, November 1996, pp. 40-42;
an article titled xe2x80x9cMultiple Wavelengths Exploit Fiber Capacityxe2x80x9d, by Eric J. Lerner, Laser Focus World, July 1997, pp. 119-125;
an article titled xe2x80x9cAdvances in Dense WDM Push Diode-Laser Designxe2x80x9d, by Diana Zankowsky, Laser Focus World, August 1997, pp. 167-172;
an article titled xe2x80x9cMultistage Amplifier Provides Gain Across 80 nmxe2x80x9d, by Kristin Lewotesky, Laser Focus World, September 1997, pp. 22-24;
an article titled xe2x80x9cOptical switching promises cure for telecommunications logjamxe2x80x9d, by Jeff Hecht, Laser Focus World, September 1998;
The Communications Handbook, CRC Press and IEEE Press, 1997, Editor-in-Chief Jerry D. Gibson, Section 65, pp. 883-890;
an article titled xe2x80x9cWDM Local Area Networksxe2x80x9d, by Kazovsky et al., IEEE LTS, May 1992, pp. 8-15;
an article titled xe2x80x9cOptical Switches Ease Bandwidth Crunchxe2x80x9d, by Rien Flipse, EuroPhotonics, August/September 1998, pp. 44-45;
an article titled xe2x80x9cSpeed Demons: Is xe2x80x9cFaster Better and Cheaper?xe2x80x9d, by Stephanie A. Weiss, Photonics Spectra, February 1999, pp. 96-102;
an article titled xe2x80x9cWavelength Lockers Keeps Lasers in Linexe2x80x9d, by Ed Miskovic, Photonics Spectra, February 1999, pp. 104-110;
an article titled xe2x80x9cOptical switches pursue crossconnect marketsxe2x80x9d, by Hassaun Jones-Bay, Laser Focus World, May 1998, pp. 153-162;
a conference review titled xe2x80x9cOptical amplifiers revolutionize communicationsxe2x80x9d, Laser Focus World, September 1998, pp. 28-32;
an article titled xe2x80x9cCombining gratings and filters reduces WDM channel spacingxe2x80x9d, by J. J. Pan and Y. Shi, Optoelectronics World, September 1998, pp. S11-S17;
an article titled xe2x80x9cDemand triggers advances in dense WDM componentsxe2x80x9d, by Raymond Nering, Optoelectronics World, September 1998, pp. S5-S8;
an article titled xe2x80x9cOptical Networks Seek Reconfigurable Add/Drop Optionsxe2x80x9d, by Hector E. Escobar, Photonics Spectra, December 1998, pp. 163-167;
an article titled xe2x80x9cUltrafast Optical Switch Unveiledxe2x80x9d, by Michael D. Wheeler, Photonics Spectra, December 1998, pp. 42;
an article titled xe2x80x9cData express Gigabit junction with the next-generation Internetxe2x80x9d, by John C. Collins, Joel Dunn, Phil Emer and Mark Johnson, IEEE Spectrum February 1999, pp. 18-25; and
an article titled xe2x80x9cDesigning Broadband Fiber Optic Communication Systemsxe2x80x9d, by Juan F. Lam, Communication Systems Design, February 1999.
Additionally, in U.S. Pat. No. 5,170,273 to Nishio there is described a cross-talk reducing optical switching system which receives electrical digital signals at its input terminal.
U.S. Pat. No. 5,191,457 to Yamazaki describes a WDM optical communication network in which optical beams are modulated by channel discrimination signals of different frequencies.
U.S. Pat. No. 5,194,977 to Nishio describes a wavelength division switching system with reduced optical components using optical switches.
U.S. Pat. No. 5,557,439 to Alexander et al. describes wavelength division multiplexed optical communication systems configured for expansion with additional optical signal channels.
U.S. Pat. No. 5,680,490 to Cohen et al. describes a comb splitting system which demultiplexes and/or multiplexes a plurality of optical signal channels at various wavelengths.
U.S. Pat. No. 5,712,932 to Alexander et al. describes reconfigurable wavelength division multiplexed systems which include configurable optical routing systems.
U.S. patent application Ser. No. 09/126,378 filed on Jul. 30, 1998, now U.S. Pat. No. 6,404,522, describes improvements in communication performance of an optical communication system which communicates data via N different channel wavelengths using WDM.
The disclosures of all references mentioned above and throughout the present specification are hereby incorporated herein by reference.
The present invention seeks to improve performance of WDM based communication networks.
In the present invention, a network control system may be used to control and manage communication and to improve communication performance of a communication network. In a preferred embodiment of the present invention, the network control system is embodied in a node serving terminal and is employed to limit a number of channel wavelengths actually used for substantially simultaneously communicating the optical signals via a fiber optic cable to an end node to a highest number N of separable channel wavelengths at the end node. Thus, channel wavelengths that cannot be separated at the end node due to, for example, limitations of separation equipment at the end node, are not used for communication with the end node and are therefore not able to generate crosstalk among channel wavelengths or other types of interference which may degrade communication performance.
The network control system may include a passive element, such as a filter, to limit the number of channel wavelengths actually used for communication. The filter may block transmission of channel wavelengths outside a selected pass-band, or a few selected pass-bands. Channel wavelengths of optical signals carried over channel wavelengths which are not transmitted by the filter may be reflected and converted to at least one channel wavelength within at least one pass-band transmitted by the filter thereby forming converted optical signals.
The converted optical signals may be transmitted via a separate route to another node serving terminal which may communicate the converted optical signals to the end node via a separate link and a separate route.
Alternatively, the converted optical signals may be transmitted to the end node over the at least one channel wavelength within the at least one pass-band transmitted by the filter after a delay time period. The delay time period may be provided by a delay generator.
Further alternatively, the converted optical signals may be combined with non-converted optical signals carried over the N channel wavelengths and transmitted in a combined form to the end node if a data rate of data carried over each of the N channel wavelengths resulting from the combination of the converted optical signals with the non-converted optical signals does not exceed a channel data rate threshold determined, for example, by a maximum channel capacity.
The network control system may alternatively include a controller which actively limits the number of channel wavelengths actually used for communication. When the highest number N of separable channel wavelengths at the end node is determined, the controller selects N channel wavelengths which may be used for communication with the end node, and prevents communication with the end node over channel wavelengths other than the N channel wavelengths. Preferably, channel wavelengths of optical signals carried over channel wavelengths which are not selected by the controller may be converted to at least one channel wavelength selected by the controller thereby to form converted optical signals. The converted optical signals may be treated in a similar manner as mentioned above.
It is appreciated that since one of the parameters which determine communication performance of a WDM based optical communication network is the density of channel wavelengths multiplexed by WDM, limitation of a number of channel wavelengths actually used for communicating optical signals to an end node of a WDM based optical network may improve performance of the WDM based optical communication network.
In another preferred embodiment of the present invention a network control system may control and/or modify, as necessary, data rates of data carried over N channel wavelengths multiplexed by WDM and/or an arrangement of the data carried over the N channel wavelengths in order to reduce interference and improve performance of a communication network.
Preferably, the network control system includes a controller which determines a first distribution of the N channel wavelengths in which K out of the N channel wavelengths are each characterized by the following characteristics: (a) each of the K channel wavelengths carries data at a data rate which is higher than a threshold; and (b) each of the K channel wavelengths is spaced apart a channel spacing S from at least one of the K channel wavelengths which is a nearest neighbor. The controller then produces a second distribution of the N channel wavelengths in which the number of channel wavelengths having the above mentioned characteristics is less than K. Then a router in the network control system reroutes the data from the N channel wavelengths of the first distribution to the N channel wavelengths of the second distribution. Thus, at least some of the channel wavelengths which carry data at data rates higher than the threshold are spaced apart a channel spacing which is greater than S thereby reducing interference among channel wavelengths carrying data at data rates higher than the threshold.
Alternatively, the network control system may reduce a data rate of data carried over selected channel wavelengths. A sequence of the N channel wavelengths is preferably determined in an increasing or a decreasing order so that an even channel number or an odd channel number is assigned to each of the N channel wavelengths. Then, data rates may be reduced either for data carried over channel wavelengths having the odd channel numbers or for data carried over channel wavelengths having the even channel numbers, in each case with respect to data rates of data carried over channel wavelengths which are nearest neighbors. Thus, channel wavelengths carrying data at data rates which are not reduced, i.e., channel wavelengths having even channel numbers or channel wavelengths having odd channel numbers respectively, are less interfered.
Further alternatively, the network control system may determine data rates carried over each of the N channel wavelengths, and equalize data rates carried over channel wavelengths in at least a sub-group of the N channel wavelengths with respect to each other in the sub-group up to a data rate difference level within a predetermined equalization range. Since a quality of communication over a channel wavelength is dependent upon a data rate of data carried over the channel wavelength, equalization of data rates carried over the at least a sub-group of the N channel wavelengths may be used to improve the quality of communication.
The network control system may be also employed to control transmission of optical signals multiplexed by WDM to an end node of the communication network. Preferably, first optical signals carried over K1 channel wavelengths, and second optical signals carried over K2 channel wavelengths which are different from the K1 channel wavelengths are received at routing apparatus. However, only the K1 channel wavelengths are used for communication with the end node.
Preferably, a wavelength converter unit which is operatively associated with the routing apparatus changes the K2 channel wavelengths of the second optical signals to at least one of the K1 channel wavelengths thereby to form converted optical signals. Then, a transmitter unit which is operatively associated with the routing apparatus, transmits the converted optical signals to the end node in one of the following transmission modes: (a) together with the first optical signals and over the K1 channel wavelengths if a combination of the converted optical signals and the first optical signals does not exceed a channel data rate threshold on each of the K1 channel wavelengths, and (b) after transmission of the first optical signals and over the at least one of the K1 channel wavelengths if a combination of the converted optical signals and the first optical signals exceeds the channel data rate threshold on the at least one of the K1 channel wavelengths.
There is thus provided in accordance with a preferred embodiment of the present invention a network control method for use with a communication network in which a node serving terminal (NST) communicates optical signals multiplexed by WDM with an end node via a fiber optic cable, the method including determining a highest number N of separable channel wavelengths at the end node, and limiting a number of channel wavelengths actually used for substantially simultaneously communicating the optical signals via the fiber optic cable to the end node to the number N.
Preferably, the determining step includes the steps of obtaining the number of channel wavelengths N at one of the NST and the end node upon installation of channel wavelength separating equipment at a service area of the end node, and providing the number of channel wavelengths N to the NST if the number of channel wavelengths N is obtained at the end node. Alternatively, the determining step preferably includes the step of transmitting control information including a determination of the number N from the end node to the NST.
Preferably, the limiting step includes the step of filtering channel wavelengths routed to the end node to enable substantially simultaneous transmission of no more than N channel wavelengths in total within at least one bandwidth. Additionally, the method also includes the step of converting at least one channel wavelength outside the at least one bandwidth to at least one of the N channel wavelengths.
The limiting step may also preferably include the steps of transmitting optical signals carried over up to N channel wavelengths from the NST to the end node, changing channel wavelengths of optical signals carried over channel wavelengths other than the N channel wavelengths to at least one of the N channel wavelengths thereby to form converted optical signals, and transmitting the converted optical signals over the at least one of the N channel wavelengths from the NST to the end node after the step of transmitting optical signals carried over up to N channel wavelengths from the NST to the end node.
Alternatively, the limiting step may preferably include the steps of determining N channel wavelengths to be actually used for substantially simultaneously communicating the optical signals to the end node via the fiber optic cable, changing channel wavelengths of optical signals carried over channel wavelengths other than the N channel wavelengths to at least one of the N channel wavelengths thereby to form converted optical signals, combining the converted optical signals carried over the at least one of the N channel wavelengths and optical signals carried over the N channel wavelengths to form combined optical signals, and transmitting the combined optical signals over the N channel wavelengths from the NST to the end node.
There is also provided in accordance with a preferred embodiment of the present invention a network control method for use with a communication network utilizing WDM of N channel wavelengths carrying data at various data rates and having the channel wavelengths spaced apart a channel spacing S from each other, the method including determining a first distribution of the N channel wavelengths in which K out of the N channel wavelengths are each characterized by the following characteristics: (a) each of the K channel wavelengths carries data at a data rate which is higher than a threshold, and (b) each of the K channel wavelengths is spaced apart the channel spacing S from at least one of the K channel wavelengths which is a nearest neighbor, producing a second distribution of the N channel wavelengths in which the number of channel wavelengths having the characteristics is less than K, and communicating the data over the N channel wavelengths of the second distribution.
In accordance with another preferred embodiment of the present invention there is also provided a network control method for use with a communication network utilizing WDM of N channel wavelengths carrying data at various data rates, the method including detecting an interference between at least two of the N channel wavelengths, determining a sequence of the N channel wavelengths having odd channel wavelengths and even channel wavelengths, and reducing one of the following: (a) a data rate of data carried over at least one odd channel wavelength in the sequence of the N channel wavelengths with respect to a data rate of data carried over even channel wavelengths which are nearest neighbors to the at least one odd channel wavelength in the sequence of the N channel wavelengths, and (b) a data rate of data carried over at least one even channel wavelength in the sequence of the N channel wavelengths with respect to a data rate of data carried over odd channel wavelengths which are nearest neighbors to the at least one even channel wavelength in the sequence of the N channel wavelengths.
Preferably, the reducing step includes the step of reducing the data rate until interference between the at least two of the N channel wavelengths falls below an acceptable interference level.
Further in accordance with a preferred embodiment of the present invention there is provided a network control method for use with a communication network utilizing WDM of N channel wavelengths carrying data at various data rates, the method including determining data rates carried over each of the N channel wavelengths, and equalizing data rates carried over channel wavelengths in at least a sub-group of the N channel wavelengths with respect to each other in the sub-group up to a data rate difference level within a predetermined equalization range.
Preferably, the equalizing step includes the step of transferring data from at least a first channel wavelength in the sub-group which carries data at a first data rate to at least a second channel wavelength in the sub-group which carries data at a second data rate being lower than the first data rate until a difference between a data rate of data carried over the at least a first channel wavelength and a data rate of data carried over the at least a second channel wavelength is within the predetermined equalization range.
There is also provided in accordance with a preferred embodiment of the present invention a network control system in a node serving terminal which communicates optical signals multiplexed by wavelength division multiplexing with an end node via a fiber optic cable, the network control system including routing apparatus capable of substantially simultaneously communicating the optical signals over a plurality of channel wavelengths via the fiber optic cable, and limiting apparatus, operatively associated with the routing apparatus, and operative to limit a number of channel wavelengths actually used by the routing apparatus for substantially simultaneously communicating the optical signals to the end node via the fiber optic cable to a highest number N of separable channel wavelengths at the end node.
Preferably, the limiting apparatus includes a bandpass filter substantially transparent only to optical signals transmitted over up to N channel wavelengths in total within at least one bandwidth. Alternatively, the limiting apparatus includes a controller operative to select N channel wavelengths from the plurality of channel wavelengths, and to prevent routing of channel wavelengths other than the N channel wavelengths together with the N channel wavelengths.
Additionally, the limiting apparatus also includes one of the following routers: an N-channel router operatively associated with the controller and operative to substantially simultaneously route up to N channel wavelengths, and a router operatively associated with the controller and operative to substantially simultaneously route a number of channel wavelengths determined by the controller.
Preferably, the system also includes a wavelength converter unit operative to convert at least one of the plurality of channel wavelengths not included in the N channel wavelengths to at least one of the N channel wavelengths.
In accordance with a preferred embodiment of the present invention there is also provided a network control system in a communication network utilizing WDM of N channel wavelengths carrying data at various data rates and having the channel wavelengths spaced apart a channel spacing S from each other, the system including a controller including: determination apparatus operative to determine a first distribution of the N channel wavelengths in which K out of the N channel wavelengths are each characterized by the following characteristics: (a) each of the K channel wavelengths carries data at a data rate which is higher than a threshold, and (b) each of the K channel wavelengths is spaced apart the channel spacing S from at least one of the K channel wavelengths which is a nearest neighbor, and a processing unit, operatively associated with the determination apparatus, and operative to produce a second distribution of the N channel wavelengths in which the number of channel wavelengths having the characteristics is less than K, and a router operatively associated with the controller and operative to reroute the data from the N channel wavelengths of the first distribution to the N channel wavelengths of the second distribution.
There is also provided in accordance with another preferred embodiment of the present invention a network control system in a communication network utilizing WDM of N channel wavelengths carrying data at various data rates, the system including a detector unit operative to detect an interference between at least two of the N channel wavelengths, determination apparatus operative to determine a sequence of the N channel wavelengths having odd channel wavelengths and even channel wavelengths, and a processing unit operatively associated with the determination apparatus and the detector unit and operative to cause a reduction of one of the following: (a) a data rate of data carried over at least one odd channel wavelength in the sequence of the N channel wavelengths with respect to a data rate of data carried over even channel wavelengths which are nearest neighbors to the at least one odd channel wavelength in the sequence of the N channel wavelengths, and (b) a data rate of data carried over at least one even channel wavelength in the sequence of the N channel wavelengths with respect to a data rate of data carried over odd channel wavelengths which are nearest neighbors to the at least one even channel wavelength in the sequence of the N channel wavelengths.
Further in accordance with a preferred embodiment of the present invention there is also provided a network control system in a communication network utilizing WDM of N channel wavelengths carrying data at various data rates, the system including determination apparatus operative to determine data rates carried over each of the N channel wavelengths, and a processing unit operatively associated with the determination apparatus and operative to equalize data rates carried over channel wavelengths in at least a sub-group of the N channel wavelengths with respect to each other in the sub-group up to a data rate difference level within a predetermined equalization range.
In accordance with yet another preferred embodiment of the present invention there is also provided a network control system for controlling transmission of optical signals multiplexed by WDM to an end node of a communication network, the system including routing apparatus operative to receive first optical signals carried over K1 channel wavelengths, and second optical signals carried over K2 channel wavelengths which are different from the K1 channel wavelengths, a wavelength converter unit operatively associated with the routing apparatus and operative to change the K2 channel wavelengths of the second optical signals to at least one of the K1 channel wavelengths thereby to form converted optical signals, and a transmitter unit operatively associated with the routing apparatus and operative to transmit the converted optical signals to the end node in one of the two following transmission modes: (a) together with the first optical signals and over the K1 channel wavelengths if a combination of the converted optical signals and the first optical signals does not exceed a channel data rate threshold on each of the K1 channel wavelengths, and (b) after transmission of the first optical signals and over the at least one of the K1 channel wavelengths if a combination of the converted optical signals and the first optical signals exceeds the channel data rate threshold on the at least one of the K1 channel wavelengths.
There is also provided in accordance with yet another preferred embodiment of the present invention a network control method for controlling transmission of optical signals multiplexed by wavelength division multiplexing to an end node of a communication network, the method including receiving first optical signals carried over K1 channel wavelengths, and also receiving second optical signals carried over K2 channel wavelengths which are different from the K1 channel wavelengths, changing the K2 channel wavelengths of the second optical signals to at least one of the K1 channel wavelengths thereby to form converted optical signals, and transmitting the converted optical signals to the end node in one of the two following transmission modes: (a) together with the first optical signals and over the K1 channel wavelengths if a combination of the converted optical signals and the first optical signals does not exceed a channel data rate threshold on each of the K1 channel wavelengths, and (b) after transmission of the first optical signals and over the at least one of the K1 channel wavelengths if a combination of the converted optical signals and the first optical signals exceeds the channel data rate threshold on the at least one of the K1 channel wavelengths.
In accordance with a preferred embodiment of the present invention there is provided a delay generator including an optical converter operative to convert incoming optical signals to electronic signals, a circular buffer operatively associated with the optical converter and operative to receive the electronic signals and to output electronic signals delayed by a delay time period, and an electronic converter operatively associated with the circular buffer and operative to convert the electronic signals delayed by the delay time period to a delayed replica of the incoming optical signals.
Further in accordance with a preferred embodiment of the present invention there is provided a delay generator for delaying optical signals, the delay generator including a plurality of delay segments connected in series, each delay segment including: a fiber optic cable of predetermined length operative to pass the optical signals, a beam-splitter coupled to the fiber optic cable and operative to transmit and reflect the optical signals at selected proportions, and a coupler for coupling the delay segment to a fiber optic cable of an adjacent delay segment, and a segment selector for directing optical signals emanating from a beam-splitter of a selected segment to an output port.